When manufacturing integrated circuits, silicon dioxide (SiO.sub.2) layers are commonly used, in particular for insulating successive conductive layers from the bulk of the integrated circuits. Those silicon dioxide layers are generally formed from a deposition of silicon dioxide on the whole surface of an integrated circuit wafer, this layer being then etched by various photo-etching methods including a chemical etch or a plasma etch. The edges of the remaining silicon dioxide layer portions after the etching are generally very stiff and form steps. This can cause, when further depositing a metallic layer such as an aluminum layer, a poor overlap of the edges and possibly metal breakings and accordingly an interruption of the conductive circuits. Therefore, one wishes to round the edges of the silica layer portions. This is generally carried out by curing at a temperature higher than the vitrous-transition temperature of the silica for causing its flowing. Very often, for reducing the temperature of this thermal step, a "dopant" such as boron or phosphorous is incorporated into silica.
It is wishable to reduce as much as possible the time duration of the flowing thermal step. This duration has to be long enough for obtaining a wished rounding of the edges and must not be too long. On the one hand, every thermal step during the manufacturing of an integrated circuit has an influence on the steps formerly carried out on the circuit; in particular, it causes a diffusion of the dopants formerly diffused into the semiconductive layers and modifies the configuration of the active areas. On the other hand, an excessive flowing could impair the operation of the circuits for example due to a large reduction of the insulator thickness on some raised portions of the underlying layers.
Here is the problem that the invention aims to solve. Indeed, thin doped silica layers obtained for example by chemical vapor deposition (CVD) are not highly reproducible, as regards their composition, in particular during plasma enhanced deposition. As the glass viscosity is very dependent upon its doping level, small variations in the silica composition from a wafer to another and from a manufacturing batch to another will cause important variations in the flowing speed and accordingly in the obtained configurations.
Thus, presently, for determining the time duration of the flowing step, an X-ray analysis or a chemical analysis of the doping level of a silica layer is carried out, but those analysis give only a general value of the doping level while only the boron or phosphorous solved in the form of B O or P O have an action on the vitrous transition temperature. Therefore, it is not possible to obtain a measuring permitting to derive precise values of the thermal step.
Another method consists in making a test on a reference wafer, carrying out a flowing operation, then cleaving the wafer and observing the shape of the steps through an electronic microscope. Then the thermal step of the actual plate is carried out with the optimal value obtained from the reference wafers. This method presents the drawback of necessiting a reference wafer and carrying out thereon a plurality of operations.
Therefore, an object of the instant invention is to provide for a method permitting to measure in situ the flowing of a layer of a material and a device for implementing this method.